Designed cellular coal

ABSTRACT

Coal-based cellular products that can be custom designed to have integral stiffeners or load paths, directed heat transfer paths, and/or directed mass transfer paths and methods for their production are described. Such design and production is made possible by the appropriate selection of starting materials, thermal treatment conditions and mold materials combined in at least some instances with segregation of different starting materials in different regions of a forming mold.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseother on reasonable terms as provided for by the terms of contract no.N00014-00-C-0062 awarded by Office of Naval Research.

FIELD OF THE INVENTION

The present invention relates to cellular coal products formed from coaland to their methods of production and more particularly to suchproducts that are custom manufactured to provide regions and volumeswithin their structures that exhibit varying strength, density and orporosity characteristics.

BACKGROUND OF THE INVENTION

U.S. patent application Ser. No. 09/453,729, filed Dec. 2,1999 entitled“Cellular Coal Products and Processes” describes coal-based cellular orporous products having a density of preferably between about 0.1 g/cm³and about 0.8 g/cm³ that are produced by the controlled heating of coalparticulate preferably up to 1 mm in diameter in a “mold” and under anon-oxidizing atmosphere. The porous products produced by theseprocesses, preferably as a net shape or near net shape, can be machined,adhered and otherwise fabricated to produce a wide variety of low cost,low density products, or used in its preformed shape as a filter, heator electrical insulator etc. Such cellular products have been shown toexhibit compressive strengths of up to about 4000 psi. Impregnation withappropriate materials or the incorporation of various strength improvingadditives can further increase the compressive, tensile and otherproperties of these cellular materials. Further treatment bygraphitization yields cellular products that can be used as electricalor heat conductors.

The coal-based products described in the foregoing U.S. PatentApplication have many potential uses that are supported by their lowdensities, high strengths, thermal and electrical conductivities andfluid permeabilities, however, many such applications or uses requirecombinations of such properties in order to provide the desired product.For example, while low density and relatively high strength may berequired for a specified use, attachment of the coal-based foam productdemonstrating such low density and relatively high strength may requirethe inclusion of a very high strength region in a portion of the productpart to permit the passage of a fastener that will induce areas of highmechanical stress in the area of fastener passage. Similarly, whilefluid permeation of the coal-based cellular product may be desired inone direction, it may be necessary to limit or inhibit such fluidpermeation in orthogonal directions to achieve directed fluid flow asthrough a “pipe”. It can also be envisioned that a low-densitycoal-based product part may require the inclusion of high strengthregions to provide structural properties in regions thereof that aresubjected to elevated directional mechanical stresses that the bulk ofthe low density material cannot adequately support.

Thus, it would be highly desirable to be able to custom design variousregions of the coal-based cellular product to support various differentstrength, thermal and electrical conductivity and fluid permeationproperties in the various regions of the coal-based cellular product.Stated differently, it would be highly desirable to possess the abilityto provide coal-base cellular products and parts produced from a singlecell forming process that exhibit integral: 1) stiffeners or load paths;2) directed heat or electrical transfer paths; and 3) directed masstransfer paths.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor the production of coal-based cellular products and parts thatexhibit integral, custom designed and varying strength, thermal andelectrical conductivity and fluid permeabilities throughout theiroverall volumes.

SUMMARY OF THE INVENTION

According to the present invention, coal-based cellular products havingintegral stiffeners or load paths, directed heat transfer paths anddirected mass transfer paths are provided through the placement ofcoal-based cells of a different size and/or density than those making upthe matrix of the product during manufacture. There is also provided amethod for the production of coal-based cellular products possessingthese characteristics. The method of the present invention utilizes theability to select and design such properties through the properselection and control of cell size and density. Such control of cellsize and density is in turn achieved through appropriate selection ofstarting materials, starting material particle size, mold packing andprocessing parameters.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a horizontally graded coal-base cellularproduct.

FIG. 2 is a photomicrograph of a vertically graded coal-based cellularproduct.

FIG. 3 is a photomicrograph of a mixed precursor coal-based cellularproduct.

FIG. 4 is a photomicrograph of a radially graded coal-based cellularproduct.

FIG. 5 is a photographic mage of a coal-base cellular productincorporating differing cell sizes in the shape of the letter T.

DETAILED DESCRIPTION

According to the method described in aforementioned U.S. patentapplication Ser. No. 09/453,729, a low density, i.e., from about 0.1 toabout 1.0 g/cm³, and preferably from about 0.2 to about 0.5 g/cm³,coal-based cellular product is produced from powdered coal particulatepreferably less than about 1 mm in diameter by the controlled heating ofthe powdered coal in a “mold” under a non-oxidizing atmosphere. Thestarting material coal may include bitumen, anthracite, or even lignite,or blends of these, but is preferably bituminous, agglomerating coalsthat have been comminuted to an appropriate particle size, preferably toa fine powder below about −60 to −80 mesh. As used herein, the term“coal-based” is meant define that the cellular products described hereinare prepared or manufactured by the “controlled swelling” of ground orcomminuted coal.

These cellular coal-based products are semi-crystalline or moreaccurately turbostratically-ordered and largely isotropic i.e.,demonstrating physical properties that are approximately equal in alldirections, and typically exhibit pore sizes on the order of less than100 μm, although pore sizes of up to 2000 μm are possible within theoperating parameters of the process described. The thermalconductivities of the cellular coal-based products are generally lessthan about 1.0 W/m/° K. Typically, the cellular coal-based products ofthe present invention demonstrate compressive strengths on the order offrom about 2000 to about 4000 psi at densities of from about 0.4 toabout 0.5 g/cm³.

The production method previously described comprises: 1) heating a coalparticulate of preferably small i.e., less than about 200 μm particlesize in a “mold” and under a non-oxidizing atmosphere at a heat up rateof from about 1 to about 20° C. per minute to a temperature of betweenabout 300 and about 700° C.; 2) soaking at a temperature of betweenabout 300 and 700° C. for from about 10 minutes up to about 12 hours toform a preform or finished product; and 3) controllably cooling thepreform or finished product to a temperature below about 100° C. Thenon-oxidizing atmosphere may be provided by the introduction of inert ornon-oxidizing gas into the “mold” at a pressure of from about 0 psi,i.e., free flowing gas, up to about 500 psi. The inert gas used may beany of the commonly used inert or non-oxidizing gases such as nitrogen,helium, argon, CO₂, etc.

It is generally not desirable that the reaction chamber be vented duringthe heating and soaking operation. The pressure of the chamber and theincreasing volatile content therein tends to retard furthervolatilization while the cellular product sinters at the indicatedelevated temperatures. If the furnace is vented during soaking, aninsufficient amount of volatile matter may be present to permitinter-particle sintering of the coal particles thus resulting in theformation of a sintered powder as opposed to the desired cellularproduct.

Additional more conventional blowing agents may be added to theparticulate prior to expansion to enhance or otherwise modify thepore-forming operation.

The term “mold”, as used herein is meant to define a device forproviding controlled dimensional forming of the expanding coal. Thus,any chamber into which the coal particulate is deposited prior to orduring heating and which, upon the coal powder attaining the appropriateexpansion temperature, contains and shapes the expanding porous coal tosome predetermined configuration such as: a flat sheet; a curved sheet;a shaped object; a building block; a rod; tube or any other desiredsolid shape can be considered a “mold” for purposes of the presentinvention. Mold materials include glass and ceramics as well as aluminumand steel. As will be explained more completely below, the selection ofmold material and consequently heating/volitization rates can affectcell formation and product properties and is consequently an importantconsideration in the production of the custom designed foams of thepresent invention.

As will be apparent to the skilled artisan familiar with pressurized gasrelease reactions, as the pressure in the reaction vessel increases,from 0 psi to 500 psi, as imposed by the non-oxidizing gas, equilibriumwill be shifted and the density of the produced coal-based cellularproduct will increase as the size of the “bubbles” or pores produced inthe expanded coal decreases. Similarly, a low soak temperature of, forexample about 400° C. will result in a smaller pore or bubble size andconsequently a more dense expanded coal than would be achieved with asoak temperature of about 600° C. Further, the heat-up rate will alsoaffect pore size, a faster heat-up rate resulting in a larger pore sizeand consequently a less dense expanded coal product than a slow heat-uprate. These phenomenon are, of course, due to the kinetics of thevolatile release reactions which are affected, as just described, by theambient pressure and temperature and the rate at which that temperatureis achieved as well as the material composition of the mold. Theseprocess variables can be used to custom produce the expanded coals ofthe present invention in a wide variety of controlled densities,strengths etc.

After expanding the coal particulate as just described, the porous coalproduct is an open celled material. Several techniques have beendeveloped for “sealing” the surface of the open celled structure toimprove, for example, its adhesive and permeability characteristics forfurther fabrication and assembly of a number of parts or for particularutilities. For example, a layer of a commercially available graphiticadhesive can be coated onto the surface and cured at elevatedtemperature or allowed to cure at room temperature to provide anadherent skin.

After expanding, the porous coal-based preform or product is readilymachineable, sawable and otherwise readily fabricated using conventionalfabrication techniques.

Subsequent to production of the preform or product as just described,the preform or product may be subjected to carbonization and/orgraphitization according to conventional processes to obtain particularproperties desirable for specific applications. Activation, for example,by ozone or carbon dioxide, may also be performed, if activation of thecoal-based expanded product would be useful in a final productapplication such as in filtering of air. Additionally, a variety ofadditives and structural reinforcers may be added to the coal-basedpreforms or products either before or after expansion to enhancespecific mechanical properties such as fracture strain, fracturetoughness and impact resistance. For example, particles, whiskers,fibers, plates, etc. of appropriate carbonaceous or ceramic compositioncan be incorporated into the porous coal-based preform or product toenhance its mechanical properties.

The open celled, coal-based preforms or products can additionally beimpregnated with, for example, petroleum pitch, epoxy resins, phenolicresins or other polymers using a vacuum assisted resin transfer type ofprocess. The incorporation of such additives provides load transferadvantages similar to those demonstrated in carbon composite materials.In effect a 3-D composite is produced that demonstrates enhanced impactresistance and load transfer properties.

The cooling step in the expansion process results in some relativelymimor shrinkage on the order of less than about 5% and generally in therange of from about 2% to about 3%. This shrinkage must be accounted forin the production of near net shape preforms or final products ofspecific dimensions and is readily determinable through trial and errorwith the particular coal starting material being used. The shrinkage maybe further minimized by the addition of some inert solid material suchas coke particles, ceramic particles, ground waste from the coalexpansion process etc. as is common practice in ceramic fabrication.

Carbonization is conventionally performed by heating the preform orproduct under an appropriate inert gas at a heat-up rate of less thanabout 10° C. per minute to a temperature of between about 800° C. andabout 1200° C. and soaking for about 1 hour or less. Appropriate inertgases are those described above that are tolerant of these hightemperatures. The carbonization process serves to remove all of thenon-carbon elements present in the preform or product such as sulfur,oxygen, hydrogen, etc.

Graphitization, commonly involves heating the preform or product eitherbefore or after carbonization at heat-up rate of less than about 20° C.per minute, preferably from about 1° C. to about 5° C. per minute, to atemperature of between about 1700° C. and about 3000° C. in anatmosphere of helium or argon and soaking for a period of less thanabout one hour. Again, the inert gas may be supplied at a pressureranging from about 0 psi up to a few atmospheres.

Through previous work performed in connection with above-referenced U.S.patent application Ser. No. 09/453,729 which is hereby incorporated byreference in its entirety, it has been determined that the cell size ofcoal-base cellular products is largely determined by the particle sizeof the starting carbonaceous material. Large cells are produced fromlarge particle size precursors while smaller cells are produced fromsmall particle size precursors. Additionally, it has been discoveredthat density can be graded or varied by both selection of differentprecursors or starting materials as well as thermal gradients within themold. The latter, as will be shown, below can be controlled by theselection of mold materials to accelerate or decelerate heat up and cooldown rates.

Coals suitable for use in the processes described herein are primarilybituminous coals exhibiting a “swell index” as determined by ASTMstandards DD5515-97, “Standard Test Method for the Determination ofSwelling Properties of Bituminous Coal” and D720-91 “Standard TestMethod for Free Swelling Index of Coal” of between about 3 and about 9and preferably about 4. Best results are achieved in terms of adequatecell generation to obtain coal-based cellular materials of the properdensities when the bituminous coal demonstrates a Gieseler plasticityvalue commonly characterized as high, i.e. above about 500 DDPM. Suchvalues are determined in accordance with ASTM standard D-2639.Agglomerating bituminous coals, i.e. those containing from about 10 toabout 32% by weight volatiles are specifically preferred.

In the first proposed application of the processes of the presentinvention, the overall weight of foams for structural applications canbe optimized by locating cellular material of high-density or largercell wall thickness in regions of high stress, while retaining the lowerdensity or more open-celled structure in less stress-critical regions.This approach can also be used to alter mass properties such as centroidmovement, moment of inertia, and radius of gyration of a solid body.

For heat transfer, two methods of design are possible. First, regions ofhigh density or larger cell wall thickness can be included to offergreater heat conduction in desired locations or directions with lowdensity cellular material located to insulate or retard heat flow inother locations or directions. Second, in convective or fluid flowapplications, regions of larger, more open celled material can belocated to allow gas flow and interaction with graphitic cell walls andligaments, while higher density or more closed cell material can be usedto prevent convective flow in other locations or directions. Such amaterial could find application in heat exchange applications, forexample. These latter materials are also applicable to directed masstransfer applications, for example, in catalysis and electrolytetransfer. In these case, coal-based cellular materials can be designedto allow preferential reactant or electrolyte transfer in a givendirection(s) to optimize the efficiency of fuel cells, batteries,catalytic and electrochemical processes. Hence, the coal-based cellularproducts of the present invention provide the integration of a cellularstructure demonstrating a particular strength, heat transfer or masstransfer characteristic in a coal-based cellular matrix of acompositionally similar but characteristically dissimilar coal-basedcellular structure.

Coal-based cellular materials of this type, hereinafter called “gradedmaterials” are produced by segregating starting materials havingdifferent particle size distributions using partitions placed in themold during loading. The partitions can then be removed prior to heatingwithout material displacement and the different cell sizes therebysegregated in the chosen regions. Alternatively, the partitions maycomprise materials such as paper that are vaporized during the heatingoperation, or even carbon meshes or fabrics that become incorporatedinto the structure during heating.

Density can be similarly graded by two distinct means; 1) use ofdifferent precursors and 2) the proper and controlled application ofthermal gradients. In the first method, different starting materials aresegregated by partitions in the loading operation as just described. Forexample, part of the mold can be filled with a high volatile bituminouscoal and the balance of the mold filled with a low volatile bituminouscoal. In this situation, the mass loss and sintering behavior of the twodifferent starting materials can be quite different resulting in gradeddensity within the structure.

In the second proposed application of the processes of the presentinvention, thermal conditions during cell formation are used to createdensity gradients within the formed article. For example when cellformation is performed in a thermally insulating mold (glass or ceramic,for example) under rapid heat up conditions, volatiles are largelyremoved from the top or outer surface areas before the interior materialreaches temperature. The surface material thus becomes dense due tobeing pushed upward or outward by the more slowly expanding internalmaterial. A product formed in such a process will have a dense skin anda less dense core. Thermal conditioning can also be used to produce theopposite effect. If a suitable material is rapidly heated to a very hightemperature and/or if the mold is made of a thermally conductivematerial such as steel or aluminum, the material adjacent to the moldsurface devolatilizes quickly and becomes rigid, i.e. cokes and remainsmore porous, while the internal structure is being formed and cannot“collapse” the outer surface during expansion as in the case of the slowheat up rate process in an insulating mold just described.

The invention will be better understood when considered in light of thefollowing, non-limiting examples of its implementation.

EXAMPLES Example 1

A high volatile bituminous coal was ground and sieved to produce twodistinct particle size ranges; 1) that passing through a 60 mesh screen,and 2) that passing through a 30 mesh screen but not a 35 mesh screen.Equal amounts of both sizes were weighed and poured onto opposite sidesof a paper partition that had been placed across the diameter of acircular ceramic mold. After vibrating the mold to achieve properparticle packing, the partition was removed and the mold loaded into apressure vessel. The mold under nitrogen pressure of 500 psi was heatedto 475° C. at a heating rate of 3° C. per minute and held at thistemperature for seven hours. The resulting cellular product was calcinedat 1000° C. and then cut to reveal the graded cell structure shown nFIG. 1.

Example 2

A high volatile bituminous coal (610 from Pike County, Kentucky) wasground until it passed through an 80 mesh screen, loaded into a steelmold and placed in a pressure vessel containing nitrogen at atmosphericpressure. While this material is known from previous experience toproduce a uniform density cellular material when subjected totemperatures between about 425° C. and 475° C., it was heated rapidly to525° C. and held at this temperature for seven hours. The resultingstructure, showing a low density inner portion and a high densitysurface is shown in FIG. 2.

Example 3

A low volatile bituminous coal was ground until all material passedthrough a 60-mesh screen. Similarly, a high volatile bituminous coal(610 from Pike County, Kentucky) was ground until it passed through an80-mesh screen. Equal amounts of both precursors were weighed and pouredon opposite sides of a paper partition placed across the diameter of acircular ceramic mold. After vibrating the mold to achieve properpacking, the partition was removed and the mold loaded into a pressurevessel containing nitrogen at 500 psi. The mold was heated to 475° C. ata slow heating rate (2° C. per minute) and held at this temperature forseven hours. The resulting cellular material was cut to reveal thegraded cellular structure shown in FIG. 3.

Example 4

An NMP (n-methyl pyrollidone) extract of a low volatile bituminous coalwas ground until all material passed trough a 60-mesh screen. It wasloaded into a ceramic mold and heated slowly to a temperature of 475° C.and held for seven hours. The exterior material volatalized first andwas pushed outward to the mold surface by the expanding interior givingrise to the pronounced “skin-core” structure shown in FIG. 4.

Example 5

A high volatile bituminous coal was ground a sieved to produce towdistinct particle sizes. The first (1) passed through a 60 mesh screenand the second (2) through a 35 mesh but not a 60 mesh screen. AT-shaped mold was constructed from paper and centered in a cylindricalsteel mold. The relative areas of the T and the surrounding mold wereused to determine the amounts of each material used. In one case,fraction 1 was loaded into the T and fraction 2 into the surroundingmold. In the second case, fraction 2 was loaded into the T and fraction1 into the surrounding mold. Both were heated to 475° C. at a heatingrate of 2° C. per minute and held at this temperature for seven hours.The resulting cellular materials were cut to reveal the linearly-gradedcell structure shown in FIGS. 5 and 6 respectively.

There have thus been described, coal-based cellular products that can becustom designed to have integral stiffeners or load paths, directed heattransfer paths, and/or directed mass transfer paths and methods fortheir production. Such design and production is made possible by theappropriate selection of starting materials, thermal treatmentconditions and mold materials combined in at least some instances withsegregation of different starting materials in different regions of theforming mold.

As the invention as been described, it will be apparent to those skilledin the art that the same can be varied in many ways without departingfrom the sprit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1. A method for the production of a coal-based cellular productcomprising a matrix of cells having cells of a different densitycomprising: selecting a first coal-based precursor ground to a particlesize below about 1 mm for providing a matrix of an appropriate strengthand density; selecting a second coal-based precursor ground to aparticle size below about 1 mm, having a different particle size thanthat of said matrix, said second coal-based precursor when expandedproviding at least one of an integral stiffener, a load path, a heattransfer path, and a mass transfer path; loading each of said first andsecond coal-based precursors into first and second volumes of a mold,wherein first and second coal-based precursors are separated bypartitions within the mold; heating said mold under a non-oxidizingatmosphere to a temperature ranging from about 300° C. to about 700° C.and soaking at this temperature for a period of from about 10 minutes toabout 12 hours; and controllably cooling said coal-based product.
 2. Themethod for the production of a coal-based cellular product comprising amatrix of cells having cells of a different density of claim 1, whereinsaid partitions are removed prior to said heating.
 3. The method for theproduction of a coal-based cellular product comprising a matrix of cellshaving cells of a different density of claim 1, wherein said partitionsremain in place during said heating and are integrated into thecoal-based product.
 4. The method for the production of a coal-basedcellular product comprising a matrix of cells having cells of adifferent density of claim 1, wherein said mold comprises glass orceramic.
 5. A coal-based cellular product, comprising: a matrix of cellshaving a density; and at least one of an integral stiffener, load path,direct heat transfer path, and a mass transfer path coal-based cellsarranged in said matrix of cells defined by the presence of coal-basedcells having a different density and cell size than said cells of thematrix.
 6. The coal-based cellular product of claim 5, wherein saidcoal-based cellular product is formed from bituminous coal.
 7. Thecoal-based cellular product of claim 6, wherein said bituminous coal hasa swell index of from about 3 to about
 5. 8. The coal-based cellularproduct of claim 7, wherein said bituminous coal has a Gieselerplasticity value above about 500 DDPM.
 9. The coal-based cellularproduct of claim 5, wherein said coal-based cells have a greater wallthickness than said matrix cells.
 10. The coal-based cellular product ofclaim 5, wherein said coal-based cells have a greater heat conductionthen said matrix cells.
 11. The coal-based cellular product of claim 5,wherein said coal-based cellular product is a heat exchanger material.12. The coal-based cellular product of claim 5, wherein said coal-basedcells have different mass transfer characteristics than said matrixcells.
 13. A method for forming a coal-based cellular product having amatrix of cells of different densities, comprising: arranging acoal-based precursor having particle sizes below about 1 mm into athermally conductive mold; arranging said mold into a pressure chamberunder a non-oxidizing atmosphere; heating said thermally conductive moldto a temperature ranging from about 300° C. to about 700° C. and holdingat this temperature for about 10 minutes to about 12 hours; andcontrollably cooling said coal-based product for forming a coal-basedproduct having a matrix of cells of different densities.
 14. The methodfor forming a coal-based cellular product having a matrix of cells ofdifferent densities of claim 13, wherein said mold includes at least oneof aluminum and steel.
 15. The method for forming a coal-based cellularproduct having a matrix of cells of different densities of claim 13,wherein said matrix of cells have a linearly graded structure.
 16. Themethod for forming a coal-based cellular product having a matrix ofcells of different densities of claim 13, wherein said coal-basedprecursor includes a high volatile bituminous coal and a load lowvolatile bituminous coal for forming a matrix of cells having a gradedcellular structure.
 17. The method for forming a coal-based cellularproduct having a matrix of cells of different densities of claim 16,further comprising: separating said high volatile bituminous coal fromsaid low volatile bituminous coal within the mold with a removablepartition.
 18. The method for forming a coal-based cellular producthaving a matrix of cells of different densities of claim 16, furthercomprising: vibrating the mold to achieve a predetermined packingdensity of the high volatile bituminous coal and low volatile bituminouscoal.
 19. The method for forming a coal-based cellular product having amatrix of cells of different densities of claim 13, wherein saidnon-oxidizing atmosphere comprises nitrogen at a pressure ranging fromabout 25 to about 500 psi.